Recombination is required for the repair of many types of lesions and it can be a source of genetic diversity. DNA sequence divergence (homocology) is expected to impede recombination efficiency because of constraints inherent in the biochemical reactions. Ancillary genetic factors such as DNA-mismatch repair are also expected to affect recombination between homologous DNAs. We are examining the requirements and consequences of recombination between divergent DNAs to gain insight into mechanisms of recombination, mechanisms of chromosome rearrangements and mutation. In yeast, plasmid transformation has been used to examine recombinational repair of double-strand breaks present in plasmid DNA homologous to chromosomal sequences. We have been using this approach to examine recombinational repair between homologous DNA sequences in wild-type S. cerevisiae. We have found that both reciprocal and non-reciprocal recombination occurs at lower frequency than for homologous DNA. Proficiency for DNA mismatch repair appears to have little or no effect on the frequency or products (examined at the molecular level) of these recombination events. We are extending this analysis to defects in other DNA repair functions. To further investigate recombination between homologous DNAs, we have constructed, in vitro, heteroduplex plasmids containing regions of extensive mismatch which resemble structures proposed as intermediates in recombination between diverged DNAs. We are examining the actions of DNA-mismatch repair systems (MMR) on these plasmids in E. coli, in which the DNA-mismatch repair system is well characterized, and in yeast. "Homologous" plasmids can survive transformation in MMR deficient E. coli strains at near 100% efficiency; in MMR-proficient strains, these plasmids are established at lower efficiency (50%) with concommitant correction of mismatches in a processive fashion in survivors.